Membrane electrode assembly with a catalyst layer including an inorganic oxide catalyst carrier and a highly hydrophobic substance and solid polymer fuel cell using the assembly

ABSTRACT

A membrane-electrode assembly including a catalyst layer that includes a catalyst-supporting carrier in which a catalyst is supported on a carrier made of an inorganic oxide, and a highly hydrophobic substance having a higher degree of hydrophobicity than the inorganic oxide, the catalyst layer being formed on at least one surface of a polymer electrolyte membrane. It is preferable that, in the membrane-electrode assembly, the degree of hydrophobicity of the highly hydrophobic substance is from 0.5 vol % to 45 vol % at 25° C., the degree of hydrophobicity being defined as a concentration of methanol (vol %) when a light transmittance of a dispersion obtained by dispersing the highly hydrophobic substance in a mixed solution of water and methanol reaches 80%.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.15/300,075, now U.S. Pat. No. 10,355,285, which was the U.S. nationalstage of International application PCT/JP2015/056522, filed Mar. 5,2015.

TECHNICAL FIELD

The present invention relates to a membrane-electrode assembly and apolymer electrolyte fuel cell using the membrane-electrode assembly.

BACKGROUND ART

A polymer electrolyte fuel cell has a structure in which catalyst layersare arranged on respective surfaces of a polymer electrolyte membrane,and a gas diffusion layer is arranged on the outer side of each catalystlayer. In general, the catalyst layer is a porous layer made of acatalyst-supporting carrier in which a noble metal catalyst is supportedon the surface of carrier particles. A fuel gas such as hydrogen ormethanol, or an oxidant such as oxygen or air, circulates through theporous layer, and an electrode reaction occurs at the three-phaseinterface, producing water within the catalyst layer.

The produced water dissipates from the catalyst layer, but in somecases, the water accumulates within the catalyst layer. If thisprogresses, the catalyst layer will not be able to hold the watertherein, giving rise to a phenomenon called flooding. With the aim ofpreventing flooding, Patent Literature 1 proposes to provide a cathodecatalyst layer of a fuel cell with sections for reducing oxygen andsections having a higher water repellency than the oxygen-reducingsections. When the surface of the cathode catalyst layer is observed,the highly water-repellent sections are distributed unevenly.

The fuel cell disclosed in Patent Literature 1 employs a carbon materialas a catalyst carrier. In cases where, for example, a fuel cell is usedso as to be activated and stopped repeatedly in a short time, carbonmaterials are known to oxidize and corrode during voltage fluctuation orduring the stoppage of supplied gas. With the aim of overcoming thisdrawback of carbon materials, proposals have been made to use materialsother than carbon materials, such as metal oxides, as the catalystcarrier.

The surface of a metal oxide, however, has high wettability to water,which makes flooding more likely compared to carbon materials. PatentLiterature 2 provides a carrier made of an inorganic substance withwater repellency by causing a water-repellent surface protectionsubstance to be adsorbed on the carrier surface. Examples of thewater-repellent surface protection substance include long-chain organicacids such as stearic acid, silica-based materials, and fluorine-basedmaterials.

CITATION LIST Patent Literature

Patent Literature 1: JP 2004-171847A

Patent Literature 2: JP 2009-099486A

SUMMARY OF INVENTION

The technique disclosed in Patent Literature 2 provides a carrier withwater repellency, which helps to prevent flooding. Providing waterrepellency to a carrier, however, tends to affect the catalystsubstances supported on the carrier.

An objective of the invention is to provide a membrane-electrodeassembly and a polymer electrolyte fuel cell capable of overcoming thevarious drawbacks of the aforementioned conventional art.

The present invention provides a membrane-electrode assembly including acatalyst layer that includes a catalyst-supporting carrier in which acatalyst is supported on a carrier made of an inorganic oxide, and ahighly hydrophobic substance having a higher degree of hydrophobicitythan the inorganic oxide, the catalyst layer being formed on at leastone surface of a polymer electrolyte membrane.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view illustrating an embodiment of theinvention.

FIG. 2 is a graph illustrating the relationship between current densityand cell voltage in a case where air is the oxidant in each of thepolymer electrolyte fuel cells obtained in Examples 1 and 5 to 7 andComparative Example 2.

FIG. 3 is a graph illustrating the relationship between cell voltage andthe amount of highly hydrophobic substance added in each of the polymerelectrolyte fuel cells obtained in Examples 1 and 5 to 7 and ComparativeExample 2.

FIG. 4 is a graph illustrating the relationship between the cell's ohmicresistance and the amount of highly hydrophobic substance added in eachof the polymer electrolyte fuel cells obtained in Examples 1 and 5 to 7and Comparative Example 2.

FIG. 5 is a graph illustrating the relationship between O₂ gain and theamount of highly hydrophobic substance added in each of the polymerelectrolyte fuel cells obtained in Examples 1 and 5 to 7 and ComparativeExample 2.

DESCRIPTION OF EMBODIMENTS

The present invention is described below according to preferredembodiments thereof with reference to the drawings. FIG. 1 illustratesan embodiment of the present invention. A polymer electrolyte fuel cell1 illustrated in FIG. 1 includes a membrane-electrode assembly 10. Themembrane-electrode assembly 10 is constituted by arranging a cathode 2and an anode 3 on respective surfaces of a polymer electrolyte membrane4. The fuel cell 1 further includes a pair of separators 5, 5sandwiching the membrane-electrode assembly 10. This structureconstitutes a unit cell.

As illustrated in FIG. 1 , the cathode 2, the anode 3, and theelectrolyte membrane 4 have the same shape, such as a rectangular shape,and have substantially the same size. As illustrated in FIG. 1 , theseparators 5 are bipolar separators as referred to in the art, and eachseparator has, on the surface opposing the membrane-electrode assembly10, a plurality of rib-shaped projections 5 a extending along onedirection. The rib-shaped projections 5 a are arranged with intervalstherebetween, and the intervals are substantially even. On the surfaceof the separator 5 opposing the cathode 2 of the membrane-electrodeassembly 10, the spaces between adjacent projections 5 a constituteoxidant supplying sections 20 for supplying an oxidant to the cathode 2.On the other hand, on the surface of the separator 5 opposing the anode3 of the membrane-electrode assembly 10, the spaces between adjacentprojections 5 a constitute fuel gas supplying sections 30 for supplyingfuel to the anode 3.

The cathode 2 has: a catalyst layer (not illustrated) that is locatedadjacent to one surface of the electrolyte membrane 4; and a gasdiffusion layer (not illustrated) that is located adjacent to thecatalyst layer. The catalyst layer includes a carrier on which acatalyst is supported. The same applies to the anode 3.

The carrier included in the catalyst layer of the cathode 2 isconstituted by particles of an inorganic oxide. In addition to thecarrier, the catalyst layer also includes particles of a highlyhydrophobic substance having a higher degree of hydrophobicity than theinorganic oxide constituting the carrier. In the present invention, the“degree of hydrophobicity” refers to a numerical value serving as ayardstick of hydrophilicity/hydrophobicity, and can be measured asfollows by using a powder wettability tester (WET101P from RhescaCorporation). A highly hydrophobic substance is crushed and loosened inadvance and is sifted through a sieve with 250-μm openings, and 50 mg ofthis highly hydrophobic substance is added to 60 ml (temperature: 25°C.) of water and stirred with stirring blades. In this state, methanolis added dropwise into the water, and the methanol aqueous solution isirradiated with a laser beam having a wavelength of 780 nm, and thetransmittance is measured. As the highly hydrophobic substance gets wet,it settles down and becomes suspended; the concentration-by-volume ofmethanol when the transmittance reaches 80% is defined as the “degree ofhydrophobicity”. It is judged that, the greater this value is, thehigher the level of hydrophobicity. Because the catalyst layer includessuch a highly hydrophobic substance, the highly hydrophobic substanceacts so as to easily dissipate water produced within the catalyst layerto the outside of the catalyst layer and effectively prevent flooding,even in cases where the catalyst layer includes a carrier constituted byparticles of an inorganic oxide which has a relatively highhydrophilicity.

Further, Inventors' study results have revealed that the level ofhydrophilicity/hydrophobicity of the inorganic oxide can be expressedmore accurately also by the “degree of hydrophilicity” defined below, inaddition to the degree of hydrophobicity defined as above. In thepresent invention, the “degree of hydrophilicity” refers to a numericalvalue serving as a yardstick of hydrophilicity, and is defined as aratio (water vapor adsorption amount/BET specific surface area) betweenthe water vapor adsorption amount (cm³/g) and the BET specific surfacearea (m²/g) found from nitrogen gas adsorption.

The specific surface area for calculating the aforementioned degree ofhydrophilicity is generally measured by employing physical adsorption ofe.g. nitrogen gas, and is measured, for example, by the BET method. TheBET equation is expressed as 1/cv₁+(c−1)p/cv₁p₀, where c and v₁ areconstants. From this equation, it is possible to estimate the specificsurface area from the intercept and slope of p/p₀ and the density of theadsorbate. Specific surface area measurement by the BET method can bespecifically performed by using, for example, SA3100 from BechmanCoulter or Flowsorb II from Micromeritics. More specifically, BET_(N2)(m²/g) is measured, for example, according to the following method.Approximately 0.3 g of a measurement sample is placed in a sample celland is pre-treated in a nitrogen atmosphere at 105° C. for 6 hours. Thespecific surface area of the treated sample is found according to theBET method.

The water vapor adsorption amount is found from an adsorption isothermusing water vapor as the adsorbate. More specifically, the water vaporadsorption amount can be measured by using 3ΔFlex from Micromeritics.More specifically, for example, approximately 0.2 g of a measurementsample is placed in a sample cell and is pre-treated in a vacuum at 105°C. for 6 hours. An adsorption isotherm is created for the treated samplein a 25° C. constant temperature oven while changing the water vaporpartial pressure p/p0 from 0.001 to 0.9. The water vapor adsorptionamount per unit mass of the sample when p/p0 is 0.9 is estimated fromthe obtained adsorption isotherm.

Inventors' study results have revealed that a lower hydrophilicity ofthe highly hydrophobic substance, i.e., a higher hydrophobicity, doesnot necessarily lead to the effect of preventing flooding. In contrast,study results have revealed that, in cases where a highly hydrophobicsubstance is included in a catalyst layer that uses, as the carrier, acarbon material which is a material having a relatively highhydrophobicity, the higher the hydrophobicity of the highly hydrophobicsubstance, the more preferable. So, in cases of using, as the carrier,an inorganic oxide which is a material having a relatively highhydrophilicity, it is advantageous, from the viewpoint of effectivelypreventing flooding, to set the level of hydrophobicity of the highlyhydrophobic substance, which is to be included in the catalyst layerwith the carrier, within an appropriate range. From this viewpoint, thedegree of hydrophobicity of the highly hydrophobic substance ispreferably from 0.5 vol % to 45 vol %, more preferably from 0.5 vol % to39 vol %, even more preferably from 0.7 vol % to 35 vol %, and furthermore preferably from 1.0 vol % to 30 vol %.

On the other hand, as regards the degree of hydrophilicity, the degreeof hydrophilicity of the highly hydrophobic substance is preferably from0.002 cm³/m² to 0.48 cm³/m², more preferably from 0.002 cm³/m² to 0.30cm³/m², even more preferably from 0.002 cm³/m² to 0.13 cm³/m².

The degree of hydrophobicity demanded of the highly hydrophobicsubstance is also related to the degree of hydrophobicity of the carrierincluded in the catalyst layer. The degree of hydrophobicity of thehighly hydrophobic substance is higher than the degree of hydrophobicityof the carrier, as described above, and it is preferable that the degreeof hydrophobicity of the highly hydrophobic substance is preferably from0.5 vol % to 39 vol % higher, more preferably from 1.0 vol % to 30 vol %higher, than the degree of hydrophobicity of the carrier. Setting thedegree of hydrophobicity of the highly hydrophobic substance within thisrange can prevent flooding more effectively. Herein, “the degree ofhydrophobicity of the carrier” refers to the degree of hydrophobicity ina state where no catalyst is supported on the carrier.

Similarly, also regarding the degree of hydrophilicity, the degree ofhydrophilicity of the highly hydrophobic substance is also related tothe degree of hydrophilicity of the carrier included in the catalystlayer. It is preferable that the degree of hydrophilicity of the highlyhydrophobic substance is preferably from 0.35 cm³/m² to 0.83 cm³/m²lower, more preferably from 0.53 cm³/m² to 0.83 cm³/m² lower, than thedegree of hydrophilicity of the carrier. Setting the degree ofhydrophilicity of the highly hydrophobic substance within this range canprevent flooding more effectively. Herein, “the degree of hydrophilicityof the carrier” refers to the degree of hydrophilicity in a state whereno catalyst is supported on the carrier.

Inventors' study results have revealed that, even when a highlyhydrophobic substance is included in the catalyst layer of the cathode2, the internal resistance of the fuel cell does not change. This meansthat the highly hydrophobic substance does not function as anelectroconductive adjuvant, but the highly hydrophobic substancecontributes only to the effective prevention of flooding owing toimproved gas diffusibility. In contrast, the aforementioned PatentLiterature 1 describes that the inclusion of a highly hydrophobicsubstance in a catalyst layer using a carbon material as a carrierblocks the gas diffusion paths in the catalyst layer.

The highly hydrophobic substance is preferably selected from substancesthat do not affect the electrode reaction of the cathode 2. It is alsopreferable to select the highly hydrophobic substance from substancesthat are chemically stable even when the cathode 2 is exposed to highelectric potential. Examples of highly hydrophobic substances includecarbon powders, such as graphite carbon black (referred to hereinafteralso as “GCB”), graphitized carbon, graphitized acetylene black, andacetylene black. It is preferable that the carbon powder's crystallitesize Lc (002) in the c-axis direction as calculated from a (002)diffraction line (ICSD card number 00-056-0159) measured by powder X-raydiffractometry is from 1 nm to 10 nm. Particularly, as for the carbonpower, it is preferable to use GCB which is an electrochemically stablesubstance owing to high crystallinity.

In the present invention, in place of GCB, or in addition to GCB, it ispossible to use, as the highly hydrophobic substance, a powder of alater-described inorganic oxide, which is used as the carrier, whosesurface is covered with a hydrophobic material. Examples of suchinorganic oxides include indium-containing oxides, tin-containingoxides, titanium-containing oxides, zirconium-containing oxides,selenium-containing oxides, tungsten-containing oxides, zinc-containingoxides, vanadium-containing oxides, tantalum-containing oxides,niobium-containing oxides, and rhenium-containing oxides. Morepreferable examples of inorganic oxides include tin-doped indium oxide,and metal-doped or nonmetal-doped tin oxides, such as antimony-doped tinoxide, fluorine-doped tin oxide, tantalum-doped tin oxide, andniobium-doped tin oxide. Among the aforementioned inorganic oxides, itis particularly preferable to use a tin-containing oxide from theviewpoint of acid resistance and high electron conductivity. Examples ofhydrophobic materials for covering the surface of the inorganic oxidepowder include fluorine-containing polymer compounds. An example of thefluorine-containing polymer compound includes atetrafluoroethylene-hexafluoropropylene (FEP) copolymer. Other examplesof hydrophobic materials include silicon-containing compounds. Examplesof silicon-containing compounds include alkoxysilanes, e.g.hexyltriethoxysilane and fluoroalkoxysilanes such asnonafluorohexylmethoxysilane, and silazanes such ashexamethyldisilazane.

The amount of the highly hydrophobic substance that may be included inthe catalyst layer of the cathode 2 can be set over a wide range. Morespecifically, the proportion of the highly hydrophobic substance withrespect to the total volume of the catalyst-supporting carrier and thehighly hydrophobic substance is preferably from 4 vol % to 63 vol %,more preferably from 16 vol % to 63 vol %, even more preferably from 16vol % to 39 vol %. By setting the proportion of the highly hydrophobicsubstance within this range, flooding can be prevented effectively, andsignificant occurrence of dry-up can be effectively suppressed duringlow-humidity operation of the fuel cell.

A catalyst may unintentionally and unavoidably exist on the surface ofthe highly hydrophobic substance; however, from the viewpoint ofsuppressing degradation of the highly hydrophobic substance, it ispreferable that no catalyst is intentionally supported on the surface ofthe highly hydrophobic substance.

As described above, the highly hydrophobic substance has a particulateform. The primary particle diameter of the highly hydrophobic substanceparticles is preferably from 10 nm to 500 nm, more preferably from 10 nmto 300 nm, even more preferably from 10 nm to 100 nm. So long as theprimary particle diameter is within the aforementioned range, the highlyhydrophobic substance particles may exist in the catalyst layer in theform of secondary particles made by the agglomeration of primaryparticles. The primary particle diameter of the highly hydrophobicsubstance is found by: observing the cross section of the catalyst layerwith an electron microscope; measuring the maximum transverse length ofat least 500 particles; and calculating the average value thereof.

As described above, an inorganic oxide is used for the carrier includedin the catalyst layer of the cathode 2. For the inorganic oxide, it ispossible to use, for example, a metal oxide, a nonmetal oxide, or asemimetal oxide. The inorganic oxide may or may not have electronconductivity. From the viewpoint of increasing the electron conductivityof the catalyst layer of the cathode 2, it is preferable that theinorganic oxide has electron conductivity. For example, it is preferableto use an inorganic oxide having a volume resistivity of 1 MΩ cm orless. Examples of inorganic oxides include indium-based oxides,tin-based oxides, titanium-based oxides, zirconium-based oxides,selenium-based oxides, tungsten-based oxides, zinc-based oxides,vanadium-based oxides, tantalum-based oxides, niobium-based oxides, andrhenium-based oxides. More preferable examples of inorganic oxidesinclude tin-doped indium oxide, and metal-doped or nonmetal-doped tinoxides, such as antimony-doped tin oxide, fluorine-doped tin oxide,tantalum-doped tin oxide, and niobium-doped tin oxide. In cases ofusing, among the aforementioned inorganic oxides, a tin oxide or a tinoxide doped with a metal such as niobium or tantalum, it is preferableto use GCB for the highly hydrophobic substance. In cases of doping atin oxide with a metal M such as niobium or tantalum, the amount of thedopant metal M, as expressed by M (mol)/[M (mol)+Sn (mol)]×100, ispreferably from 0.01 at. % to 20 at. %, more preferably from 0.1 at. %to 10 at. %.

From the viewpoint of allowing the catalyst to be supported over a largespecific surface area, the primary particle diameter of the carrier ispreferably from 5 nm to 200 nm, more preferably from 5 nm to 100 nm,even more preferably from 5 nm to 50 nm. The method for measuring theprimary particle diameter may be the same as the method for measuringthe primary particle diameter of the highly hydrophobic substanceparticles. The BET specific surface area of the carrier is preferablyfrom 20 m²/g to 1500 m²/g.

As for the catalyst to be supported on the carrier made of an inorganicoxide, it is possible to use any catalyst similar to those usedheretofore in the present technical field, with examples includingplatinum, alloys of platinum and a noble metal other than platinum (suchas ruthenium, rhodium, or iridium), and alloys of platinum and a basemetal (such as vanadium, chromium, cobalt, nickel, iron, or titanium).From the viewpoint of exerting catalytic activity efficiently, it ispreferable that the average particle diameter of the catalyst on thesurface of the carrier is from 1 nanometer to several dozen nanometers.

There is no particular limitation to the method for fixing the catalystonto the carrier surface, and it is possible to employ any methodsimilar to those known heretofore in the present technical field. Forexample, when platinum is used as the catalyst, platinum can be fixed tothe carrier by employing, for example, platinic chloride hexahydrate(H₂PtCl₆.6H₂O) or dinitrodiamine platinum (Pt(NH₃)₂(NO₂)₂) as a platinumsource, and reducing the same by a known method, such as liquid-phasechemical reduction, gas-phase chemical reduction, impregnation-reductionpyrolysis, a colloidal method, or surface-modified colloid pyrolysisreduction. The amount of catalyst supported is preferably from 1 mass %to 70 mass % with respect to the mass of the carrier.

In addition to the aforementioned substances, the catalyst layer of thecathode 2 may include, as necessary, materials similar to those knownheretofore in the present technical field, such as an ionomer and abinder for binding particles.

The cathode 2 of the membrane-electrode assembly 10 has been describedabove, but the anode 3 may be configured similarly to the cathode 2.Note, however, that flooding is less likely to occur in the catalystlayer of the anode 3 than in the catalyst layer of the cathode 2, andthus, it is not essential to include the aforementioned highlyhydrophobic substance in the catalyst layer of the anode 3. Also, thecatalyst layer of the anode 3 is not exposed to high electric potentialduring the operation of the fuel cell, and thus, it is not essential touse an inorganic oxide as the catalyst carrier. Thus, it is possible touse, for example, a carbon material as the catalyst carrier.

The cathode 2 and the anode 3 can be formed by applying, on the surfaceof the polymer membrane 4 or the gas diffusion layer, a mixtureincluding, for example, a catalyst-supporting carrier, a highlyhydrophobic substance (in case of the cathode 2), an ionomer, and asolvent in which the ionomer can be dissolved. Any method known in thepresent technical field can be employed for the formation means withoutparticular limitation. For example, it is possible to employ generallyknown coating means, such as screen print coating, doctor blade coating,spray coating, slit die coating, reverse coating, or bar coating.

For the polymer electrolyte membrane 4, it is possible to use a materialthat is chemically stable in the environment inside the fuel cell andthat has high proton conductivity. Further, for the polymer electrolytemembrane 4, it is preferable to use a material that has no electronconductivity and is less likely to cause gas crossover. A preferredexample of such a material is a polymer electrolyte membrane in which asulfonic acid group is bonded to a fluoro-polyethylene main chain. Otherusable examples include polysulfones, polyether sulfones, polyetherether sulfones, polyether ether ketones, and materials obtained bysulfonating a hydrocarbon-based polymer.

For the gas diffusion layer, it is preferable to use a material havingelectron conductivity and having a structure that is capable ofdiffusing, to the respective catalyst layers of the cathode 2 and theanode 3, fuel gas and oxidant through the oxidant supplying sections 20and the fuel gas supplying sections 30. For the aforementioned material,a porous member made primarily of a carbon-containing material can beused. More specifically, it is possible to use a porous carbon materialformed by carbon fibers, such as carbon paper, carbon cloth, and carbonnonwoven fabric. The aforementioned usable materials may be subjected toa surface treatment, such as a water-repellent treatment or hydrophilictreatment.

The material for the separator 5 is not particularly limited, so long asit has electron conductivity and is capable of forming oxidant supplyingsections 20 and fuel gas supplying sections 30. Examples of suchmaterials include metals such as stainless steel, carbon, and mixturesof carbon and resin.

EXAMPLES

The present invention is described in further detail below according toExamples. The scope of the present invention, however, is not limited tothe following Examples. Unless particularly stated otherwise, “%” refersto “mass % (percent by mass)”.

Example 1

(1) Preparation of Ink for Cathode Catalyst:

Particles of Sn_(0.96)Nb_(0.04)O_(x) (expressed as SnO₂:Nb), in whichSnO₂ was doped with 4 at. % of Nb (number of moles with respect to Snand Nb), were used as the carrier. The carrier was produced according tothe method described in WO 2009/060582, and had a structure in which aplurality of particles were connected moniliformly. When the degree ofhydrophobicity of the carrier was measured according to theaforementioned method, the carrier settled down in water before methanolwas added. Thus, the degree of hydrophobicity was 0. To this carrier, 7%of platinum was fixed according to the colloidal method described in JP9-167622A. Then, 1.5 g of the platinum-supporting fine particles wasplaced in a container containing 3-mm-dia. zirconia beads, andpredetermined amounts of water and ethanol were further added to thiscontainer, to obtain a slurry. Using a planetary ball mill, the slurrywas mixed at 270 rpm for 30 minutes. Next, a 5% Nafion (registeredtradename) solution (from DuPont) was added such that the volume ratio(Nafion/carrier) was 0.67, and mixing was continued at 270 rpm for 30minutes, to form an ink.

Separate from the aforementioned operation, GCB having a specificsurface area of 150 m²/g and an average particle diameter D₅₀ of 12.3 μmwas used as the highly hydrophobic substance, and 0.063 g of the GCB wassampled in a container. Water and ethanol were poured into thiscontainer, and the mixture was subjected to ultrasonic dispersion for 30minutes. The ink prepared according to the aforementioned method wasadded to the thus-obtained dispersion, and the mixture was further mixedat 270 rpm for 30 minutes, to prepare an ink for the cathode catalyst.The employed GCB's crystallite size Lc (002) in the c-axis direction ascalculated from a (002) diffraction line measured by powder X-raydiffractometry was 2.95 nm, with 0.89 for the Scherrer constant.

(2) Preparation of Ink for Anode Catalyst:

A commercially available platinum-supporting carbon black (TEC10E50Efrom Tanaka Kikinzoku) was used as the catalyst-supporting carrier. Nohighly hydrophobic substance was used. Except for these points, an inkfor an anode catalyst was prepared in the same way as in (1) above.

(3) Formation of Catalyst Layer:

The cathode catalyst ink and the anode catalyst ink were applied torespective surfaces of a Nafion (registered tradename) electrolytemembrane (product number NRE212 from DuPont) with a Pulse Swirl Spraydevice from Nordson. The amount of platinum applied was adjusted to 0.1mg/cm² on the cathode surface and to 0.5 mg/cm² on the anode surface.The ink was pressed with a hot press at 140° C. and 10 kgf/cm² for 3minutes. In this way, a catalyst-layer-coated electrolyte membrane(Catalyst Coated Membrane; CCM) was prepared.

(4) Preparation of Polymer Electrolyte Fuel Cell:

The thus-obtained CCM was sandwiched between a pair of gas diffusionlayers (product number 25BCH from SGL Carbon). This was furthersandwiched between a pair of separators consisting of carbon plates eachhaving gas flow paths formed therein, to thereby prepare a polymerelectrolyte fuel cell having the structure illustrated in FIG. 1 . Thethus-obtained fuel cell was equivalent to a JAM standard cell.

Example 2

Si-coated SnO₂:Ta particles were used instead of GCB in (1) ofExample 1. SnO₂:Ta refers to Ta-doped SnO₂. SnO₂:Ta refers to SnO₂ thatis doped with 2.5 at. % of Ta (number of moles with respect to Sn andTa). SnO₂:Ta was produced according to the method described in WO2014/136908. The SnO₂:Ta was coated with Si. The Si-coated SnO₂:Taparticles were produced according to the following method. In acontainer were placed 6 g of SnO₂:Ta and 3 mL of methyltrimethoxysilane(product number KBM-13 from Shin-Etsu Chemical Co., Ltd.), and themixture was manually shaken at room temperature for 30 minutes. Theobtained mixed solution was gradually heated to 120° C., and wassubjected to heat condensation at 150° C. for 2 hours. In this way,particles in which the surface of SnO₂:Ta was coated with Si wereobtained. The amount of Si coated was 16.6 mg/g-SnO₂:Ta.

Further, in (3) of Example 1, the amount of platinum applied to thecathode surface was 0.07 mg/cm².

Except for the above points, a polymer electrolyte fuel cell wasprepared in the same way as in Example 1.

Example 3

FEP-coated SnO₂:Nb particles were used instead of GCB in (1) ofExample 1. SnO₂:Nb refers to SnO₂ that is doped with 4 at. % of Nb(number of moles with respect to Sn and Nb). Except for this point, apolymer electrolyte fuel cell was prepared in the same way as inExample 1. FEP is a copolymer of tetrafluoroethylene andhexafluoropropylene. The FEP-coated SnO₂:Nb particles were producedaccording to the following method.

To 10 mL of pure water was added 0.177 g of an FEP dispersion (productnumber 120-JRB from Du Pont-Mitsui Fluorochemicals Co., Ltd.), and themixture was stirred and mixed with a magnetic stirrer. To thisdispersion was added 1.5 g of SnO₂:Nb particles, and the dispersion wasstirred and mixed for 30 minutes. Then, the pH was adjusted to 2 byusing 0.1 N nitric acid, to cause the FEP to be adsorbed onto thesurface of the SnO₂:Nb particles. After filtering and rinsing, theresidue was calcined with a tube furnace in a nitrogen atmosphere at280° C. for 6 hours, to obtain FEP-coated SnO₂:Nb particles.

Example 4

Si-coated SnO₂:Nb particles were used instead of GCB in (1) ofExample 1. SnO₂:Nb refers to Sn that is doped with 4.0 at. % of Nb(number of moles with respect to Sn and Nb). The SnO₂:Nb was coated withSi. The Si-coated SnO₂:Nb particles were produced according to thefollowing method. To a mixed solution including 120 mL of ethanol and 5mL of a 0.5 vol % acetic acid aqueous solution, 3.5 mL ofhexyltriethoxysilane (product number KBM-3063 from Shin-Etsu ChemicalCo., Ltd.) was added, and the mixture was stirred and mixed with amagnetic stirrer at room temperature for 60 minutes. To the obtainedmixed solution was added 6 g of SnO₂:Nb, and the mixture was stirred andmixed with a magnetic stirrer at room temperature for 60 minutes. Thesolution was then filtered, and the filtered residue was graduallyheated to 120° C., and was subjected to heat condensation at 150° C. for2 hours. Then, 1.5 g of the obtained powder and 2.1 mL ofhexyltriethoxysilane were placed in a container and manually shaken atroom temperature for 30 minutes. The mixed solution was gradually heatedto 120° C., and was subjected to heat condensation at 150° C. for 2hours. In this way, particles in which the surface of SnO₂:Nb was coatedwith Si were obtained. The amount of Si coated was 5.1 mg/g-SnO₂:Nb.

Further, in (3) of Example 1, the amount of platinum applied to thecathode surface was 0.12 mg/cm².

Except for the above points, a polymer electrolyte fuel cell wasprepared in the same way as in Example 1.

Comparative Example 1

SnO₂:Nb particles were used instead of GCB in (1) of Example 1. Exceptfor this point, a polymer electrolyte fuel cell was prepared in the sameway as in Example 1.

Comparative Example 2

No GCB was added to the catalyst layer of the cathode in Example 1.Except for this point, a fuel cell was obtained in the same way as inExample 1.

{Evaluation 1}

The degree of hydrophobicity of the highly hydrophobic substance used inthe cathode catalyst layer in each of the polymer electrolyte fuel cellsobtained according to the Examples and Comparative Examples was measuredaccording to the aforementioned method. Also, the degree ofhydrophilicity of SnO₂:Nb and GCB was measured according to theaforementioned method. The results are shown in Table 1 below.

Hydrogen gas was supplied to the anode side of each of the polymerelectrolyte fuel cells obtained according to the Examples andComparative Examples, and also, oxygen gas or air was supplied to thecathode side. The flow rate was set such that the utilization rate ofthe hydrogen gas was 70% and the utilization rate of oxygen was 40%.These gases were first humidified with an external humidifier and thensupplied to the fuel cell. The temperature of the fuel cell was adjustedto 80° C. The humidity of the supplied gases was adjusted to a relativehumidity of 100% RH. Electric power was generated with each of the fuelcells within an application current range such that the cell voltage didnot fall below 0.3 V. The current density when air was used as theoxidant and the cell voltage was 0.4 V was measured. The results areshown in Table 1 below.

TABLE 1 Highly hydrophobic substance Fuel cell Degree of Degree ofCurrent Proportion hydrophobicity hydrophilicity density** Type (vol %)(vol %) (cm³/m²) (A/cm²) Example 1 GCB 16  2.6 0.025 0.662 Example 2Si-coated 10 31.0 — 0.165 SnO₂:Ta Example 3 FEP-coated 10 43.4 — 0.139SnO₂:Nb Example 4 Si-coated 10 44.0 0.119 0.135 SnO₂:Nb ComparativeSnO₂:Nb 10  0*¹ 0.835 0.095 Example 1 Comparative Not added — — — 0.100Example 2 *¹Immediately settled down in water before addition ofmethanol. **Oxidant was air.

Examples 5 to 7

In Example 1, the amount of GCB added to the cathode catalyst layer waschanged so that the proportion of GCB with respect to the total volumeof Pt-supporting SnO₂:Nb and GCB was 4 vol % (Example 5), 39 vol %(Example 6), and 63 vol % (Example 7). Except for these points, fuelcells were obtained in the same way as in Example 1.

{Evaluation 2}

Each of the fuel cells obtained according to Examples 1 and 5 to 7 andComparative Example 2 was operated under the same conditions as in theaforementioned Evaluation 1, and the relationship between cell voltageand current density was obtained for when the oxidant was air. Theresults are shown in FIG. 2 . Further, based on the relationship betweencell voltage and current density, the relationships between the amountof GCB added and the cell voltage, the cell's ohmic resistance, and O₂gain were obtained for when the current density was 0.2 A/cm². Theresults are shown in FIGS. 3 to 5 . “O₂ gain” is the difference in cellvoltage between a case where oxygen is used as the oxidant and a casewhere air is used as the oxidant when electric power is generated at anoxygen utilization rate of 40%. Here, the cell voltage is the voltageafter subtracting the voltage drop caused by the internal resistance(alternating-current resistance at a frequency of 10 kHz) within thefuel cell. O₂ gain is an index expressing the diffusibility of oxygen;in the present invention, it can be judged that, the smaller this value,the further flooding was reduced.

The results shown in Table 1 and FIG. 2 clearly show that the fuel cellsof Examples 1 and 5 to 7 achieve higher current densities than the fuelcells of the Comparative Examples. FIG. 3 shows that the cell voltagebecomes substantially constant when the GCB addition amount is 16 vol %or higher. The results of FIG. 4 clearly show that there is nosubstantial change in the cell's ohmic resistance of the fuel cell evenwhen the GCB addition amount is changed. Further, the results of FIG. 5clearly show that, in contrast to Comparative Example 2 including noGCB, O₂ gain drops sharply by adding 4 vol % or more GCB.

INDUSTRIAL APPLICABILITY

As described in detail above, the present invention provides amembrane-electrode assembly in which flooding is effectively preventedwithout impairing the various characteristics of a fuel cell, and apolymer electrolyte fuel cell using the membrane-electrode assembly.

The invention claimed is:
 1. A membrane-electrode assembly comprising acatalyst layer that includes (i) a catalyst-supporting carriercomprising a catalyst supported on a carrier made of an inorganic oxideand (ii) a hydrophobic substance having a higher degree ofhydrophobicity than the inorganic oxide, the catalyst layer being formedon at least one surface of a polymer electrolyte membrane, and whereinthe carrier of the catalyst-supporting carrier is made of a tin oxidedoped, or not doped, with a metal, and wherein a degree ofhydrophobicity of the hydrophobic substance is from 0.5 vol % to 35 vol% at 25° C., the degree of hydrophobicity being defined as aconcentration of methanol (vol %) when a light transmittance of adispersion obtained by dispersing the hydrophobic substance in a mixedsolution of water and methanol reaches 80%, wherein the hydrophobicsubstance consists of a metal-doped or nonmetal-doped tin oxide coveredwith a fluorine-containing polymer compound or a silicon-containingcompound.
 2. The membrane-electrode assembly according to claim 1,wherein a water vapor adsorption amount/BET specific surface area valuewhich is the ratio between a water vapor adsorption amount (cm³/g) ofthe hydrophobic substance and a BET specific surface area (m²/g)measured by using nitrogen is from 0.002 cm³/m² to 0.48 cm³/m².
 3. Themembrane-electrode assembly according to claim 1, wherein a proportionof the hydrophobic substance with respect to the total volume of thecatalyst-supporting carrier and the hydrophobic substance is from 4 vol% to 63 vol %.
 4. The membrane-electrode assembly according to claim 1,wherein the hydrophobic substance comprises graphite carbon black,graphitized carbon, graphitized acetylene black, or acetylene black. 5.The membrane-electrode assembly according to claim 1, wherein thehydrophobic substance comprises an inorganic oxide powder whose surfaceis covered with a hydrophobic material.
 6. The membrane-electrodeassembly according to claim 5, wherein the inorganic oxide powderincludes a metal-doped or nonmetal-doped tin oxide, and the hydrophobicmaterial includes a silicon-containing compound.
 7. Themembrane-electrode assembly according to claim 5, wherein themetal-doped tin oxide is Ta-doped tin oxide or Nb-doped tin oxide. 8.The membrane-electrode assembly according to claim 5, wherein theinorganic oxide powder includes a metal-doped or nonmetal-doped tinoxide, and the hydrophobic material includes a fluorine-containingpolymer compound.
 9. A polymer electrolyte fuel cell comprising themembrane-electrode assembly according to claim 1, wherein the catalystlayer is employed as a cathode catalyst layer.